To evaluate a compressed air system,
you must begin at the end: What are your air requirements at the
point of use? Once you determine exactly what type and how much
air you need, you can begin to factor-in design considerations,
costs, and efficiencies.

Design Considerations

Careful planning of a compressed air
system is essential for smooth operation. System configuration
should take into consideration both your requirements and the
physical characteristics of your installation.

Air-cooled vs. Water-cooled.

Air-cooled compressors have either
integrally-mounted or separate oil and air coolers. These
coolers require adequate ventilation to perform reliably. For
water-cooled compressors, a supply of adequate pressure and
quality water must be available.

Single vs. Multiple Units.

Air compressors operate most
efficiently at full load. Depending on your usage profile, it
may be more costeffective to purchase multiple compressors to
accommodate load variations.

Sequencers.

Sequencers improve the efficiency and
reliability of multiple compressor systems. With microprocessor
controls, they can stabilize system pressure and even track each
unit's service, load, and maintenance hours.

Heat Recovery.

Recovering and using the heat
generated by an air compressor conserves energy. Waste heat has
many applications including process use, space heating, and
preheating boiler feedwater.

Aftercoolers.

These heat exchangers cool the
compressed air, thereby condensing much of the moisture for easy
removal. This prepares the air for further treatment.

Receiver Tank.

If you have widely varying compressed
air loads, you should consider a receiver tank to boost capacity
during peak periods. With a larger receiver tank, you can meet
occasional peak demand with a smaller compressor and avoid high
electrical demand charges.

Dryers.

Removing moisture from compressed air
is essential for virtually all applications. Air quality
requirements and ambient conditions will help determine the type
of dryer required.

Piping.

Pipes must be carefully sized and
arranged to minimize pressure drop and should be sloped to drain
towards a drop leg or moisture trap.

Filters, Regulators, & Lubricators.

These should be installed at the
point-of-use.

Condensate Control.

Because condensate must be expelled
from the system for reliable operation, drain traps should be
included in the system plan. Additionally, most localities
require that any oil be separated from condensate before the
water can be disposed of in the municipal system.

Booster Compressors.

These compressors will efficiently
increase air pressure from the plant system for equipment or
processes that require up to 500 psig or higher pressure.

Air Requirements

Air Quality.

There are six key levels of
compressed air quality ranging from shop air to breathing air.
The quality of air required will determine which type of
filtration and drying system is needed.

Air Capacity.

Begin with capacity requirements and
load factors for each tool and machine that will use compressed
air. These compressed air requirements are generally available
from the equipment manufacturers.

Air Pressure.

Determine the pressure required at
the point of use. Pneumatic tool manufacturers rate tool
capacities at specific pressure ratings. The minimum required
pressure can be determined by the equation:

Pr

=

Pp + PL where:

Pr

=

Minimum required pressure, psig.

Pp

=

Pressure at point of use, psig.

PL

=

Total pressure loss, psid.

Total pressure loss includes any losses at the air receiver,
dryers, centrifugal separator, particulate filter, oil removal
filter, and oil vapor adsorber, as well as piping and valves.

Once capacity and pressure
requirements are known, the air compressor size and input power
requirements can be obtained from manufacturers.

Cost of Compressed Air

You must go beyond initial cost when evaluating compressed air
systems. During the first year, operating costs for compressed
air can be 1 1/2 to 2 times the initial purchase price of the
equipment. Efficiency of the compressor and the overall system
efficiency are critical.

Electrical Expense.

As much as 70% of compressed air cost
is electrical.

Cooling Cost.

If you are considering air-cooled
compressors, factor-in the electricity used to run cooling fan
motors. If evaluating a water-cooled system, consider the
quantity and required quality of the water, as well as
treatment, electrical, and disposal costs.

Maintenance and Repair Costs.

The easier the system is to maintain,
the more you save in the long run. Can the system be maintained
and repaired by your in-house personnel, or does it require
out-of-house assistance for most problems?

Leaks and Unnecessary Demand.

Any leaks in your system will add to
your operating costs. Unnecessary use of compressed air is
wasteful and expensive.

Reliability.

An unreliable compressed air system
can be disastrous to the bottom line. A lost production day is
never made up.